This invention relates to improvements in the preparation of dicarboxylic acid anhydrides by the vapor phase oxidation of hydrocarbons. More specifically the improvement concerns the regeneration of catalyst for the catalytic oxidation of hydrocarbons to dicarboxylic anhydrides in the presence of a vanadium-phosphorus-oxygen (PVO) catalyst.
It is known that PVO catalysts produce high yields of dicarboxylic anhydrides. Although high yields of dicarboxylic anhydrides have been obtained by such procedures, it has been found that the yield of product diminishes with time due to a reduction in the selectivity of the catalyst. Several methods of regenerating the PVO catalyst in situ have been devised.
Two early patents in this area are U.S. Pat. Nos. 3,296,282 and 3,474,041 issued to Ralph O. Kerr. These patents disclose that the PVO catalyst may be regenerated and stabilized by adding specified organo-phosphorus compounds to the catalyst. The organo-phosphorus compound may be added with or without the hydrocarbon, either intermittently or continuously. It was found that during the use of the catalysts a portion of the phosphorus is removed from the catalyst, thereby disrupting the initial ratio of P:V in the catalyst, and the added phosphorus compound replaced a portion of the lost phosphorus to thereby maintain the catalyst close to its original selectivity and extending the useful life of the catalyst. A similar approach was disclosed in British Patent Specification No. 1,291,354, and U.S. Pat. Nos. 3,906,008 and 3,975,407.
Another approach to the same problem, i.e., the disruption of the ratio of P to V by phosphorus loss in the catalyst, is the removal of some of the vanadium. A process using this method is disclosed in U.S. Pat. Nos. 4,020,174; 4,098,807 and 4,094,816 issued to Partenheimer. The use of various organic and inorganic halogen compounds are disclosed to regenerate the catalyst. After treatment with alkyl halides, the patents suggest a steam treatment to cause a water gas reaction to remove residual carbon deposited by the alkyl halide. In the U.S. Pat. No. 4,098,807 the formation of the volatile vanadium halide is specifically noted, and the patentee discloses the unsuitability of PCl.sub.3 or PCl.sub.5 for the regeneration, because of the deposits of phosphorus onto the catalyst.
U.S. Pat. No. 4,111,832 discloses the use of aqueous ammonia and/or amine to at least partially dissolve the catalyst and thereafter redepositing the catalytic components from the solution.
British Patent Specification No. 1,464,198 discloses the regeneration of PVO oxidation catalysts by the addition of alkyl esters of orthophosphoric acid.
British Patent Specification No. 1,439,489 discloses the regeneration of PVO catalyst with a reducing agent such as hydrogen, carbon monoxide, methane or hydrogen sulfide.
The present invention is concerned with the regenerations wherein a phosphorus compound is added to the catalyst to increase selectivity and extend catalyst life. In the past the art has used the term "reactivate" to describe the situation wherein the treatment of the catalyst results in operation of the reaction system thereafter with higher yields than before the treatment, notwithstanding that the "activity" of the catalyst may have been reduced. The term "regeneration" is used herein to mean that a higher yield of product is obtained after treatment. The phosphorus compounds reduce the activity (the term "activity" as used herein, means the ability of the catalyst to convert hydrocarbon at a given temperature) of the catalyst while improving the selectivity. The overall result is that a higher yield (conversions.times.selectivity) is obtained at a slightly higher operating temperature (higher temperature to boost conversion which is the result of the P addition), i.e., the catalyst is regenerated and is more effective although the activity is actually reduced. The usefulness of this method of PVO catalyst treatment in a commercial maleic anhydride plant with fixed bed reactors is limited by the inability to evenly distribute the phosphorus throughout the catalyst bed.
In practice it has been found that the phosphorus compounds concentrate near the feed end of the catalyst bed. The catalyst, near the feed end of the bed, becomes more selective but less active. The catalyst further from the feed end of the catalyst bed is relatively unaffected, hence this portion remains active but loses selectivity with time in use. The result is that with only a portion of the bed receiving the optimum phosphorus treatment, yield continues to decline. The rate of decline is slower than it would be if no phosphorus treatment were applied, but faster than it would be if even distribution of the phosphorus could be obtained.
Another problem is that the amount of phosphorus addition must be limited; otherwise, the activity of the first part of the bed will become so low that a large portion of the reaction will occur in the last part of the bed. In this case the yield will decline, because the reaction will be occurring in the most nonselective part of the bed. Since the active part of the bed is now effectively shortened, the reactor temperature must be increased to maintain conversion because of the decreased contact time with active catalyst. This also leads to a loss in selectivity. It can be appreciated also, that as the site of the principal reaction (indicated by the reaction exotherm or "hot spot" in the temperature profile of the catalyst bed) moves toward the exit end of the catalyst bed, some of the reaction, which was occurring down stream of the exotherm, is actually eliminated because of the shortened bed.
It has now been found that the addition of steam following the addition of phosphorus compounds to a vanadium-phosphorus-oxygen catalyst obviates both of the noted problems. It is believed, the addition of steam to the catalyst bed which has been treated with phosphorus compounds removes a portion of the phosphorus from the feed end of the bed and redistributes it through the remainder of the bed. This results in the entire bed being treated. If a large amount of phosphorus is added or accumulated over a period of time and the feed end of the bed has been rendered inactive, steam treatment will redistribute the phosphorus, which will reduce the activity of the entire bed, but upon further steam treatment, the bed will tend to become active again. This is a proposed mechanism and the scope of the present invention is not intended to be limited thereby.